1. Field of the Invention
The present invention relates to a recording method for recording and erasing an image on a reversible heat-sensitive recording medium and a recording apparatus using such a recording method and, more particularly, to a technique for preventing occurrence of an incomplete erasion when a recording and erasure are repeated.
2. Description of the Related Art
In recent years, a reversible heat-sensitive recording medium has come into widespread use for displaying various kinds of information. Such a reversible heat-sensitive recording medium is disclosed, for example, in Japanese Laid-Open Patent Application No.4-44887. The reversible heat-sensitive recording medium can be used for repeatedly recording and erasing an image by using a material having a characteristic in which a transparency is reversibly changed by changing a temperature of the material.
FIG. 1 shows a structure of a reversible heat-sensitive recording medium using the above-mentioned material. In FIG. 1, the reversible heat-sensitive recording medium 1 comprises a base film 31 made of a polyester film, a recording layer 34 applied on the base film 31 and an over-coating layer 35 protecting the recording layer 34. The recording layer 34 is made of a high polymer/low polymer composite type recording material which comprises a high polymer material 32 and an organic low-molecular material 33 dispersed in the high polymer material 32.
FIG. 2 is a graph showing a temperature-transparency characteristic of a reversible heat-sensitive recording medium. The recording material of the recording layer 34 has a transparency which varies when it is heated or cooled as shown in FIG. 2. Specifically, when the recoding material is at a room temperature below a temperature T1, the recording material is in a white, opaque-like state. This state is referred to as a state A which is a maximum white state. When the recording material is heated from room temperature, the transparency of the recording material starts to increase at a temperature T2, and the transparency becomes maximum at a temperature T3. This state is referred to as a state B which is a maximum transparency state. The maximum transparency is maintained even when the recording material in the maximum transparency state B is cooled to room temperature. This state is referred to as a state C. Then, when the recording material in the state C in which the recoding material has maximum transparency at room temperature is heated again to a temperature T4 which is higher than the temperature T3 as indicated by a dashed line in FIG. 2, the transparency of the recording material assumes a state which is intermediate the maximum transparency state and the maximum white, opaque state. This state is referred to as a state D. When the recording material in the state D is cooled to the state A which is at room temperature T1, the recording material returns to the maximum white, opaque-like state.
The above-mentioned phenomenon occurs when the organic low-molecular material 33 reversibly changes from a poly-crystalline state to a single-crystalline state due to temperature change which results in changes in the transparency of the recording layer 34. Thus, the reversible heat-sensitive recording medium 1 can be repeatedly used for recording and erasing an image thereon.
Japanese Laid-Open Patent Application No.5-4446 discloses a recording apparatus which repeats recording and erasure of an image on the reversible heat-sensitive recording medium 1. This recording apparatus comprises a thermal recording head 5 and a thermal erasing head 6 as shown in FIG. 3. The reversible heat-sensitive recording medium 1 is conveyed by feed rollers 3 and platen rollers 4. The reversible heat-sensitive recording medium 1 being conveyed by the feed rollers 3 and the platen rollers 4 is detected by a position detecting sensor 36 which is located before the thermal erasing head 6. When the reversible heat-sensitive recording medium 1 is detected by the position detecting sensor 36, a temperature of the thermal erasing head 6 is rapidly increased to an erasing temperature at which the reversible heat-sensitive recoding medium 1 is changed to a transparent state before the reversible heat-sensitive recording medium 1 reaches the thermal erasing head 6. After the thermal erasing head 6 reaches the erasing temperature, a pulsed current is supplied to the thermal erasing head 6 so as to maintain the erasing temperature. Thus, the recording layer of the reversible heat-sensitive recording medium 1 is changed to a transparent state so as to erase a visible image formed by in the white state. Thereafter, a new visible image is formed on the reversible heat-sensitive recording medium 1 by the thermal recording head 5. The reversible heat-sensitive recording medium 1 can be used many times by repeating the above-mentioned operation. Since the thermal erasing head 6 has a small heat capacity, the thermal erasing head 6 can be rapidly heated after the reversible heat-sensitive recording medium 1 which is conveyed by the feed rollers 3 is detected by the position detecting sensor 36. Thereby, a visible image erasing unit of the recording apparatus can be miniaturized and energy used by the visible image erasing unit can be saved.
In an erasing operation performed by the thermal erasing head 6, the thermal erasing head 6 is moved while it is in contact with a surface of the reversible heat-sensitive recording medium 1. Accordingly, dirt such as a foreign material or a finger print which adheres on the surface of the reversible heat-sensitive recording medium 1 is transferred to a surface of the thermal erasing head 6. If such dirt adheres on the surface of the thermal erasing head 6, a thermal resistance between the thermal erasing head 6 and the reversible heat-sensitive recording medium 1 is increased. Thus, an amount of heat provided to the reversible heat-sensitive recording medium 1 is decreased which results in a locally incomplete erasure. In such condition, there is a problem in that a new visible image cannot be accurately recorded.